The present invention relates to an image forming apparatus and an image forming method.
Conventionally, there has been an image forming apparatus of four-cycle-transfer type. The image forming apparatus is provided with a photoreceptor for developing a toner image from a development rack containing toner in four colors, an intermediate transfer belt onto which the toner image is primarily transferred from the photoreceptor, a secondary-transfer roller that secondarily transfers the toner image from the intermediate transfer belt onto recording material, and a cleaner unit that removes the toner remaining on the intermediate transfer belt. While the intermediate transfer belt makes four rounds, the toner image having each of the colors is sequentially transferred in each round from the photoreceptor onto the intermediate transfer belt (see JP 2004-37916 A).
In the conventional image forming apparatus, however, reduction in size of the apparatus has led to reduction in circumferential length of the intermediate transfer belt and in printing time of the apparatus. This has accordingly caused necessity for the secondary-transfer roller and the cleaner unit to be brought into pressure contact with or spaced apart from the intermediate transfer belt in the primary transfer.
The apparatus has a problem that each color is misaligned on the toner images which are primarily transferred onto the intermediate transfer belt. This is caused by a change in velocity of the intermediate transfer belt because tension of the intermediate transfer belt is changed when the secondary-transfer roller and the cleaner unit are pressure-contacted with the intermediate transfer belt during the primary transfer.
On the other hand, the change in velocity of the intermediate transfer belt can be prevented by pressurization against the intermediate transfer belt with use of increase in the primary-transfer pressure contact force exerted between the photoreceptor and the intermediate transfer belt. However, the increase in the primary-transfer pressure contact force may cause such a problem that a primary-transfer toner image has a central blur disadvantageously.
Herein, the central blur is caused by a status in which a central portion of a toner image (e.g., character, line or the like) resists being transferred from the photoreceptor onto the intermediate transfer belt in the primary transfer, and a status in which a central portion of a toner image (e.g., character, line or the like) resists being transferred from the intermediate transfer belt onto the recording material in the secondary transfer.